In many different applications for example, in underground hard rock mines, it is extremely valuable to have timely and accurate knowledge of drill hole positions. Drill holes, commonly referred to as long holes (i.e. long hole drill and blast) are typically used for the placing of explosives in mining via open stoping, sub level stoping, block caving, vertical crater retreat methods, and sub level caving. It is useful in any underground mining that requires the drilling of long holes to distribute explosives through the rock or to run services through rock. There are however, parallel surface mining applications using top hammer machines where accurate survey is also necessary.
Underground mining by open or sub level stoping methods recovers the ore in open stopes, normally backfilled after being mined out. The stopes are excavated voids in the rock, typically with largest dimensions in a vertical direction. The ore body is divided into separate stopes for sub level open stope mining. Such a configuration is typically shown in FIG. 4 where the underground stopes 22 are formed using sub level drifts 23 strategically located as the base for a long hole drilling rig to drill a long hole blast pattern typically shown by radial lines 24. The ore is typically removed through trough undercuts 25 to draw points 26.
Between the stopes, ore sections are set aside for pillars to support the hanging wall. Pillars are normally shaped as vertical beams across the ore body. Horizontal sections of ore are also left to support mine workings above the producing stopes, known as crown pillars. Ensuring the stability of the surrounding rock mass influences mining efficiency favourably. The stability is strongly influenced by the accuracy and precision of the long hole drilling undertaken as part of the mining process.
Sub level drifts for long hole drilling are prepared inside the ore body, in between main levels. Drifts are strategically located as the base for the long hole drilling rig, to drill the long hole blast pattern typically shown at 24. Adherence to the drill pattern is a most important step for long hole blasting. The drill pattern specifies where blast holes are collared, depth and angle of each hole. All parameters are set with high precision for successful performance of the long hole blast. If the pattern of long holes deviates from the desired plan this can result in dilution of the ore body by drilling outside the design area, the creation of oversize broken rock caused by lower charge density between wandering holes, and Hanging Wall/Foot Wall damage hence stability issues through increased charge density.
Long holes are currently drilled as “up holes”, “down holes”, “rings” or in a “fan” pattern. Through practical working height restrictions in underground operations, such as in the sub level drifts 23, drilling rigs have short drill rod lengths and corresponding short feed and boom lengths to ensure ease of operation. In order to maximise mining efficiency, drilling sub levels are spaced as sparingly as possible resulting in a requirement for drilling holes many times the available rod length. These rods are typically between 1.2 meters and 3 meters long while the long holes may be over 60 meters in length.
Consequently each drill rig will have multiple rods available and often have an automated “carousel” of rods that can be inserted into the drill string as the bit is advanced. As the number of rods in the hole increases, the number of joints increases and the accuracy of the drilling process diminishes. To drill a hole, the first rod and bit is “collared” as close as possible to the surveyed position with the correct alignment to produce the desired hole. Once collared, the hole alignment is checked and the drilling process begins with a new rod added as the string advances in the hole.
Upon completion, the hole is flushed with water to remove cuttings and the rod is then retracted from the hole.
The existing technology to accurately survey drill holes requires a survey after completion of the hole. This is necessary because long holes are typically drilled by top hammer drills which introduce percussive force down the drill string as part of the drilling operation. Although technology to survey drill holes in real time (i.e. as part of the drilling operation) exists in applications where the drill string is not subject to top hammer conditions, it is not hitherto been possible to use survey tools in real time with top hammer drills due to the destructive nature of the percussive force in the drill string.
Although some top hammer drilling equipment manufacturers claim to complete real time survey as an onboard function, they rely on a critical assumption that the holes once commenced will always be straight. In practice this is not the case and holes may deviate significantly as their length increases. Typically, a survey using such equipment consists only of providing a hole length and direction assumed from parameters that can be recorded on the drilling rig.
The only presently available accurate survey method for operators of top hammer drills is post drilling survey which requires the lowering of a survey tool into the hole after the hole has been drilled, flushed and the rig moved on to a different hole location. This is a time consuming and costly task that may eventually identify hole characteristics but if deviation outside allowable constraints has occurred, then relies on significant corrective action being undertaken as a secondary or tertiary process after the top hammer drill rig has moved from the drilling site.
No real time survey technology exists that can withstand the down hole vibration and acceleration that is associated with a top hammer drill and ascertain the true path of the hole before completion and relay the data ultimately to decision making software.
In addition, many current systems, which rely on changes in the earth's magnetic field to determine position, cannot be accurately used in magnetic environments.